Compounded rubber stock



United States Patent COMPOUNDED RUBBER STOCK Alphonse iPechukas, Pittsfield, Mass assignortto-Columbia-Sonthern Chemical Corporation,tAllegheny County, P p ra on Q De aW e No Drawing. Application'November 4, 1952, ,Serial No. 318,135

be incorporated in .rubber and the rubber composition. 1

vulcanized with consequent production ,of ,a strong, wellbonded p o u n h p ep ation .e sucha product, it has been the usual practice to mill into the rubber a vulcan'izing agentsuchas sulphur,.gne,or;rnore vulcanizing accelerators, one or more activators, .a softener, an antioxidant, and the silica. These materials have been conventionally milled into the rubber simultaneously. Thereafter, the milled ,pro du ct ,i s;sha,ped intotdesiled form .and is vulcanized at ,conventional temperatures.

Acc rding o h pr s n invention, i has been foun that superior rubber compositions ,may be prepared by p op adiu m ut the Q de J u which .thesilieaiisintroduced into the ,ru'hbercomposition'yvithrespeet.totheofher com o ent Tha i i has be n found .thati the silica is introduced into the rubber before other.IQIativelypolar s mp nen a e int odu ed therein a d the polar compoueu s o e rubb r .Qwapeun ar introduced ther after, the r ul in pmi t, when vul anized, is foun to have sup ri r strength p operties Th exac reason for this is not known. 'The silica used appears, under the electron microscope, to comprise a mass of loosely bonded ag re at s sm lle ,na ticle which aggre may be termed fiocs. 'Best' reinfiorcernent .is believed to be achieved when the rubber penetrates the pores of By incorporating the :silica ,into the rubber before these components are introduced ,:and

thereafter introducing the accelerator 01 activator, this problem is avoided.

The term rubber, as herein used, is intended tomefer to natural rubber and the synthetic polymerspftdiolefin and diolefin copolymers with mono-olefin compounds trile copolymersand butadiene-isobutylene c pfllymers .(butyl rubber) and like synthetic elastomers Whichare derived from the polymerization ofbutadiene-1,3,2=chlorobutadiene, isoprene or the like alone or with polymerizable olefinic compounds-such as styrene, methyl meth- "s y acrylonitrile, methyl chloroacr-ylate or their equivalents.

. these fiocs. However, it seems probable that certain Polar equipm nt man .thevulauiza ism ac lerators .or activators, inhibit the ability of rubbentcpenetrete the pores of the silica fiocs.

f Sulphur cornmonly'is the primary ivulvanization agent .Whichis used .to vulcanize rubber-silica compositions.

' Typical organic accelerators which are used in the'p'roduction of silica-rubber compositions are the following: guanidirnes such as ,diorthotolyl 'guanidine, biphenyl guanidine, and trip'henyl 'guan'idine; thiur'ams such as *tetramethyl thiuram disulfide, -tetramethyl thiuram m'o'nm su'lfide, tetrabutyl th'iuram monosulfi'de, tctraethyl tliiuram disulfide, tetrarn'ethyl thiurarn'tetrastilfide, dipentamefthylene ;thiur am ;tetras ulfide, and thefllike; dithiocarbamates such as methyl zimate .i (Zinc dimethyl dithiocarbamate) ethyl zimate (zinc d ethyl dithiocarbamate), copper diethy l dithiocarbamate, zinc dibutyl dithiocarbamate, Zinc dibenzyl dithiocarbamate, selenium diethyl dithiocarbamate, and the like; tliiazoles such as benzdthiaZ-yl disulfide, Z-mercapto-benzothiazole or its *cupric salt, M- cyclohexyl-Z-benzothiaayl sulfenamide and the like; aldehyde amines such as butyraldehyde-aniline, heptaldehyde- :ahilinc, :and :.the dike; iand xanthates :such as .zinc ibutyl ixanthate .and the like.

dibutyl :tantratc, dioetyl phthalate, etc.

Antioxidants used include: tphenyl beta :naphthylamine, fiiphenyhp-phienylene diarni-ne, -di-o-to1yl ethylene vdiamine,:andnauiousrphenols. I

:Many of the above compounds are highly polar rsornei-are only slightly polar. Generally speaking, esters,-

mmines, alcohols, acids and salts are quite polar while {hydrocarbons such :as ibenzene :have little polarity. Sul- "ph-uris (essentially :nonpolar. Metal qOx'ideS, on :the other :h-and, are highly polar as are most of the above listed accelerators. The polarity of a compound may the determined readily by determining its dipole moment. In general, compounding ingredients having a dipole moment above about 0.6 electrostatic unit should not be addedto rubberuutil the silica has been milled in;

of oxygen may be used in accordance with the present invention.

The improved results are especially marked when a finely divided hydrated silica-prepared by precipitation'in aqueous ,;medium by reaction of an acid or acidic ma- -terial with a metallic silicate, such as sodium-silicate or calcium silicate,is subjected to use according tothis invention. Such products are finely divided hydrated amorphous silica which contains SiOz and bound "Water in the proportion corresponding to the formula: H20.(SiO2')z where xis a number-in emcess of '3 butirarely inexcess of about 50. Such silica normally has a surface area of about 25 to 200, preferably 50 to 175, square meters per gram, a particle size in the range of 0.015 to 0.04 micron, and may also contain small amounts of other metals but generally contains less than 2 per cent, preferably less than 1 per cent, by weight of NazO.

In silica of this character, two types of water are present. These types are termed bound water and free water. The term free water, as used herein, is intended to denote the water which may be removed from the silica pigment by heating the pigment at a tempera .ture of 105 C. for a period of 24 hours in a laboratory oven. The term bound water, as used herein, is intended to mean the amount of water which is driven off from the silica pigment by heating the pigment at ignition temperature, for example, 1000 to 1200 C., until no further water can be removed, minus the amount of free water in the pigment. The amount of free water in the pigment normally will range between by, weight of the total pigment.

Typical silica pigments of the type here contemplated 1 to per cent are described in application of Edward M. Allen, Serial No. 283,721, filed April 22, 1952, and in application of Alphonse Pechukas, Serial No. 290,536, filed May 28,

Typical methods of preparing silica of this character are as follows:

EXAMPLE I pump at 150 F.

The rates of flow were adjusted so that calcium chloridewas approximately 10 per cent in excess over the stoichiometric quantity required for reaction, and so 3 that the amount of liquid supplied to the pump was about 25 per cent below the output capacity of the pump. In

consequence, the solutions were subjected to turbulent intermixing in the pump.

The slurry of calcium silicate thus produced was introduced into a tank and sufiicient hydrochloric acid solution containing 28 per cent by weight of HCl was added, with stirring, to reduce the pH of the slurry to 2.

- Thereupon, sufficient sodium hydroxide solution containing 40 per cent by weight of NaOH was added to raise i the pH of the slurry to 7.5. The precipitated silica was recoveredby decantation and filtration, and was dried in an oven at a drying temperature of 120 to 140 C. for 12 hours. The free water content of the product was within the range of 3 to 8 per cent by weight of the pigment.

EXAMPLE II An aqueous solution of sodium silicate was prepared by diluting 5.88 liters of sodium silicate containing 298 grams per liter of SiOz as sodium composition NazO(SiO2)a.se, with sufficient water to prosilicate having the due 20.7 gallons of solution. A further solution was made by dissolving 1220 grams of calcium chloride and 320 grams of sodium chloride in 16.0 gallons of water.

Streams of these aqueous solutions were fed directly into the central area of a centrifugal pump, proportionmg the rates of flow so that calcium chloride remained in excate slurry and the slurry was thereafter boiled for about 4 hours, at which time the odor of ammonia was very faint. Thereafter, the slurry was washed and filtered,

and was dried at a temperature of about 120 C. A

4 white friable product having the following composition was produced:

Percent by Weight Twenty-six hundred gallons of sodium silicate solution containing 18 grams per liter of NaCl and 20 grams per liter of NazO as the sodium silicate, NazO(Si02)s.z, was placed in a 4000-gallon tank. Carbon dioxide gas containing 10 per cent by volume of CO2, the balance being nitrogen, was introduced into the solution over a period of 3 hours while holding the solution at 30 C. at a rate sufficient to react with all of the sodium silicate and convert 20-25 per cent of the N320 content thereof to bicarbonate. Thereafter, the resulting slurry was boiled for one hour, filtered, and washed. The filter cake was reslurried and a solution of Al2(SO t)3 181120 in quantity sufficient to introduce into the slurry /2 per cent of Al based upon the weight of SiOz in the slurry, was added to the slurry. Thereafter, the slurry was stirred briefly and enough hydrochloric acid was added to adjust the pH to 5.7. The resulting slurry was filtered and the filter cake dried.

The formulation of the rubber composition is effected by milling the silica into the dry rubber and thereafter adding some or all of the other components. The rubber should not, in any event, contain the organic accelerator.

Preferably, it should contain little or no fatty acid, such as stearic or oleic acid. On the other hand, these components frequently are present in synthetic rubber compositions, such as GR-S, and these components also are present in small amount in natural rubber. Removal of these minor quantities is not usually necessary.

After the silica has been milled into the rubber, it frequently is advantageous to permit it to age for 12 or more hours in order to achieve optimum results. While an improvement is observed without such aging before the other compounding ingredients are introduced, the aging before such introduction serves to improve the character of the composition.

Following the compounding, the rubber is formed into the desired shape and is vulcanized at the usual vulcanizing temperatures, for example, 250 to 375 F.

The following examples are illustrative:

Example A The following formulation was used in preparing a series of rubber stock:

Percent by weight Natural rubber 76.0 Darex 34 (butadiene-styrene polymer having high styrene content) 24.0 Cumar Ex 10.0 Silica 60.0 Zinc oxide 5.0 Iron oxide 6.0 Wyex 3.0 Santoflex B 0.67 Santoflex 35 0.33 Ethylae 0.8 Methyl Tuads 0.2 Sulfur 2.5 Benzoic acid 0.75 Laurex 3.0 Reogen 2.4 Water 3.4

In test No. 1, the natural rubber was milled in a Banbury mill for 3 minutes. One-half of the silica as prepared according to the process of Example I was added and the mixing was continued for 5 minutes. Thereupon,

the remainder of the silica was added and mixing Wyex 3:0 tinned for .12 minutes. This mess-r mixture sheeted Sulfur 3;,0 quickly from the mill and permitted to'age for brie week. g 1 0.2

At the end of the week, the resulting rdbb'er cein esi- DOTG 1 1.2 tion was placed in a Banburymi'll along With the Dare): 34 5 Santofle'x B L 0.67 and mixed for 2 minutes. Then the remainder of the Santoflex'35 1 1 r e a r :33 ingredients except the sulfur, Reogen (a mixture "of an all Benz'oie acid 0.75 soluble sulphonic acid and a parafin oil), and Water were Stearic acid 1 3.0 introduced and the mixin g'continued for one minute. The Triethanolamine 2.0

TABLE II Length of Out alter Flexlng (inches) v Minutes V Ssl'nip'leNo. can at Number 01' Cycles 4 s 0.20 0. 21 0. 2s 0. 27 0. 29 0. 35' 0. 40' 5 g 8.1;) 8.2% 8.36 0.27 0. 31 0. 34 0. 39 .5 0.30 0.30 0.38 0.44 6 -12 as as 2-22 as as water and Reogen were added and after a total 0f 5 iiiiii- In preparation of sample 4, the natural rubber was utes the batch was dumped and transferred to an open mill. milled in a laboratory Banbury mill for 3 minutes. One- The sulfur was then added and after the stock was well half of the silica was then added and mixing continued for blended it was sheeted into a sheet having a thickness of 5 minutes. The remainder of the silica was; added and 0.28 inch, for curing. This product is designated in the mixing continued, the total tiiii of mixing being i'iiii1- table below as sample No. 1. utes. This mixture was sheeted out and allowed to age A second sample, designated sample No. 2, was prepared for 4 days. p I I by milling together the natural rubber, Darex 34, and The aged product was placed (iii a roller rain and the Cumar Ex for 2 minutes in a Banbury. One-half of the GR- S-X HS was added slowly. The Marbon 8000 and silica was added with all of the other ingredients except thrma'irider of the dry ingredients were iiiixed in, the the Reogen, sulfur, and water, and milling was continued sulfur being mixed last. After careful mend ng", the for 5 minutes. Thereupon, the remainder of the silica tfietlianolamine was added and the stock 'sheeted off the with the waer and 'Reogen was added and milling con- 11111 to form asheeto'fOJZd inchinthickiiess'. tinued. After a total milling time of 17 minutes, the batch sample 5 was prepared byniilling'the natural rubber was dumped and transferred to an open mill where the and the GR-S-X478 in a labdiatdry Bafibiirymill for sulfur was milled in and the stock was sheeted as in the 3 minutes. one-half of the silica was added and mixcase of sample No. 1. ing continued for 5 minutes. The remainder of the silica A third sample was prepared substantially in the same 40 was added and mixing was continued, the total time of way that sample No. 1 was prepared except that the aging mixing being 12 minutes. This product was permitted was omitted. to age for 4 days.

These samples were vulcanized by heating for 10 min- The aged product was placed in the Banbury mill along utes at 307 F. They were then subjected to flex tests with the Ma-rbon 8000 and blended for 2 minutes. The according to standard established methods for determining remainder of the dry ingredients except the sulphur were flex resistance and the following results were obtained: added and mixing continued for 2 minutes. The tri- TABLE I ethanolamine was added and after one minute milling, the stock was transferred to an open roller mill. The sulfur was added and after 1 0 minutes, the stock was Length 8 5 33 Flexmg sheeted to form a sheet 0.2-8 inch in thickness. qmple No In preparation of sample 6, the natural rubber, Numbe -ofoy les GR- S-X47 8, and Marbon 8000 were placed in the Banbury and blended for 2 minutes. One-half of the silica 5,000 15,000 25,000 35,000 and all of the dry ingredients except the sulfur were added and mixing continued for 5 minutes. The balance 1 28 0,38 0,44 0,49 of the silica and the triethanolamine were added and mix- 3 8-32 823 8- ing continued, the total time of mixing being 17 minutes. Sulfur was thoroughly blended into this mixture using a roller mill and the mixture sheeted to form a It w1ll be notlced by comparing samples Nos. 1 and 3 shefit 028 inchin thickness with Sample 2 that substanfial improvement in It will be noted that the flex resistance of samples Nos. ure resulted by introduclng the silica first, the best results 4 {and 5 was Superior to that of Sample No. 6, lthus show blng obtained when the product was aged bef ing that a substantial improvement accrued by introduccorporating the additional ingredients. mg the silica first Example B 5 Example C In this example, the formulation was as follows: I11 this series the following formulation was used 1n Parts by weight prepanng the Stocks descnbed below butadienedstyrene .oopolymgr Percent by weight Zinc oxide 5.0 Natural rubber 55-0 Sulfur 3.0 GR-S-X478 (cold rubber) 25.0 Agerite powder 1.0 Marbon 8000 (high styrene copolymer) 20.0 Altax 1.2 Zinc oxide 5.0 Methyl Tllads 0.15 Hi-Sil 60.0 Silica (prepared as in Example I) 5 8. 5 Iron oxide 6.0 Ethylene glycol 35 Sample B was prepared in the normal manner by milling the GR-S with all of the other ingredients simultaneously. Sample A was prepared by milling the silica into the GR-S before introduction of the other components. Following introduction of silica into the rubber, the other components were milled in and the products vulcanized and tested for tensile and tear strengths according to standard test methods.

TABLE III Although the present invention has been described with reference to the specific details of certain embodiments thereof, it is not intended that such details shall be regarded as limitations upon the scope of the invent-ion except insofar as included in the accompanying claims.

What is claimed:

1. In a method of preparing a rubber composition containing silica, the improvement which comprises milling finely divided, hydrated, amorphous silica characterized by the presence of fiocs and having an average ultimate particle size in the range of 0.015 to 0.04 micron and a surface area of 50 to 200 square meters per gram into rubber which is essentially free from polar compounds including vulcanizing ingredients; and, after said silica is thoroughly distributed throughout said rubber, thereafter milling vulcanizing ingredients into the rubber, and vulcanizing the product. 1

' *2. The method of claim .1 wherein the silica is a finely divided silica containing the composition HzO(SiO2)1, where x is3 to 50, and having a surface area of 50 to 200 square meters per gram.

3. The process of claim 1 wherein the polar compound has a dipole moment of at least 0.6 electrostatic unit.

4. A method of preparing a rubber composition which comprises milling dry silica having a surface area of 50 to 200 square meters per gram into dry rubber which is essentially free from vulcanizin-g ingredients, allowing the rubber stock to age for a period of at least about 12 hours, milling vulcanizing ingredients into the rubber, and vulcanizing the product.

5. The method of claim 4 wherein the silica is a finely divided silica having the composition H20(Si02)a:, where x is 3 to 50, and having a surface area of 50 to 200 square meters per gram, said silica containing 2 to 10 per cent of free water.

6. A method of preparing a rubber composition which comprises milling dry pulverulent silica having a surface area of 50 to 200 square meters per gram with rubber which is essentially free from polar compounds, allowing the milled product to age for at least 12 hours, milling vulcanizing ingredients into the rubber, and vulcanizing the rubber.

References Cited in the file of this patent UNITED STATES PATENTS 12,118,601 Gerk'e et .al May 24, 1938 2,502,949 Howlett et al. Apr. 4, .1950 2,560,043! Schmidt July 10, 1951 OTHER REFERENCES Allen et al.: India Rubber World, August 1949, pages 577-581 and 586. 

1. IN A METHOD OF PREPARING A RUBBER COMPOSITION CONTAINING SILICA, THE IMPROVEMENT WHICH COMPRISES MILLING FINELY DIVIDED, HYDRATED, AMORPHOUS SILICA CHARACTERIZED BY THE PRESENCE OF FLOCS AND HAVING AN AVERAGE ULTIMATE PARICLE SIZE IN THE RANGE OF 0.015 TO 0.04 MICRON AND A SURFACE AREA OF 50 TO 200 SQUARE METERS PER GRAM INTO RUBBER WHICH IS ESSENTIALLY FREE FROM POLAR COMPOUNDS INCLUDING VULCANIZING INGREDIENTS; AND, AFTER SAID SILICA IS THOROUGHLY DISTRIBUTED THROUGHOUT SAID RUBBER, THEREAFTER MILLING VULCANIZING INGREDIENTS INTO THE RUBBER, AND VULCANIZING THE PRODUCT. 